In humans, the high affinity receptor for folate comes in four isoforms: alpha, beta, gamma, and delta. The alpha, beta and delta forms are typically bound to the membranes of cells by a glycosyl phosphatidylinositol (GPI) anchor. They recycle between extracellular and endocytic compartments and are capable of transporting folate into the cell. Soluble forms of folate receptor may be derived by the action of proteases or phospholipase on membrane anchored folate receptors.
Folate receptor alpha (also referred to as FRα, FR-alpha, FOLR-1 or FOLR1) is expressed in a variety of epithelial tissues, including those of the choroid plexus, lung, thyroid, kidney, uterus, breast, Fallopian tube, epididymis, and salivary glands. Weitman, S D et al., Cancer Res 52: 3396-3401 (1992); Weitman S D et al., Cancer Res 52: 6708-6711 (1992). Overexpression of FRα has been observed in various cancers, including lung cancer (e.g., carcinoid tumors, and non-small cell lung cancers, such as adenocarcinomas); mesothelioma; ovarian cancer; renal cancer; brain cancer (e.g., anaplastic ependymoma, cerebellar juvenile pilocytic astrocytoma, and brain metastases); cervical cancer; nasopharyngeal cancer; mesodermally derived tumor; squamous cell carcinoma of the head and neck; endometrial cancer; papillary serous and endometrioid adenocarcinomas of the ovary, serous cystadenocarcinomas of the ovary, breast cancer; bladder cancer; pancreatic cancer; bone cancer (e.g., high-grade osteosarcoma); pituitary cancer (e.g., pituitary adenomas); colorectal cancer and medullary thyroid cancer. See e.g., U.S. Pat. No. 7,754,698; U.S. Patent Application No. 2005/0232919; Intl. Publ. No. WO 2009/132081; Bueno R et al., J of Thoracic and Cardiovascular Surgery, 121(2): 225-233 (2001); Elkanat H & Ratnam M. Frontiers in Bioscience, 11, 506-519 (2006); Basal et al., PLoS ONE, 4(7):6292 (2009); Fisher R E J Nucl Med, 49: 899-906 (2008); Franklin, W A et al., Int J Cancer, Suppl 8: 89-95 (1994); Hartmann L C et al., Int J Cancer 121: 938-942 (2007); Iwakiri S et al., Annals of Surgical Oncology, 15(3): 889-899 (2008); European patent publication EP 2199796, Parker N. et al., Analytical Biochemistry, 338: 284-293 (2005); Weitman, S D et al., Cancer Res 52: 3396-3401 (1992); Saba N F et al., Head Neck, 31(4): 475-481 (2009); Yang R et al., Clin Cancer Res 13: 2557-2567 (2007). In some types of cancers (e.g., squamous cell carcinoma of the head and neck), a high level of FRα expression is associated with a poor prognosis, whereas in other types of cancers (e.g., non-small-cell lung cancers), a higher level of FRα expression is associated with a more favorable prognosis. See, e.g., Iwakiri S et al., Annals of Surgical Oncology, 15(3): 889-899; Saba N F et al., Head Neck, 31(4): 475-481 (2009).
Earlier detection of cancer improves survival rates and quality of life. To improve the likelihood of early detection and treatment, a pressing need exists for non-invasive methods for diagnosing FRα-expressing cancers and for monitoring existing FRα-expressing cancers.